1. Field of the Invention
The present invention relates to improved insulation for electrical power cables which has both weathering and track resistant properties. More specifically, the invention has to do with silane cross-linked track resistant insulation comprised of linear low density polyethylene, carbon black, an antioxidant and UV inhibitors. The principle use for this insulation is with 5 kV and low voltage non-shielded electrical power cables in airport lighting applications.
2. The Related Art
In applications where there is no insulation shield, the occurrence of the phenomenon of tracking in polymeric material used as electrical insulation has long been recognized as a source of electrical insulation failure. Generally speaking, this phenomenon results when the non-shielded insulation gradually acquires a conductive film on its surface from contamination, moisture, or polymer degradation which, when the insulation is subjected to a voltage stress, will over time allow a small amount of leakage current to be discharged and conduct along the surface of the insulation. The resulting temperature rise causes a drying out of the insulation. Once this happens the dry surface area so formed can frequently encounter electrical stress greater than the insulation air interface, thereby resulting in a spark or scintillation. The temperatures in the spark interior can quickly degrade the insulation to carbonaceous material that is highly conducting. This in turn leads to additional scintillations and material degradation resulting in premature cable failure.
Airport lighting cables have traditionally been non-shielded because they carry low voltage currents (xe2x89xa65 kV) and they are not in the proximity of airport personnel or passengers. The cable is often used in series lighting circuits for runways, control systems, and other parts of the airport. The need for track-resistant properties in such applications stems from the fact that because such cables lack an insulation shield, the cable can be prone to surface discharge. This discharge can track along the surface of the cable leading to premature cable failure.
Underground electrical cables used in airports in the United States must conform with Federal Aviation Administration (xe2x80x9cFMxe2x80x9d) technical specifications entitled xe2x80x9cSpecification for L-824 Underground Electrical Cable for Airport Lighting Circuitsxe2x80x9d (hereinafter xe2x80x9cFAA L-824xe2x80x9d), the contents of which are herein incorporated by reference. Additionally, electrical cables used in such applications must comply with the standards of the National Electrical Manufacturers Association (xe2x80x9cNEMAxe2x80x9d) and the Insulated Cable Engineers Association (xe2x80x9cICEAxe2x80x9d), namely NEMA Standards Publication No. WC 7/ICEA Publication No. S-66-524 entitled xe2x80x9cCross-linked Thermosetting Polyethylene Insulated Wire and Cable for the Transmission and Distribution of Electrical Energyxe2x80x9d, the contents of which are herein incorporated by reference. The insulation of the present invention is designed to be track resistant while also meeting the requirements of these technical specifications and standards.
Non-tracking insulations need to be resistant to water and degradation by ultraviolet (xe2x80x9cUVxe2x80x9d) light. This can commonly be accomplished by adding carbon black to the insulation; however, in airport lighting applications non-tracking insulations are even more sensitive to conductivity because they are non-shielded. That is, they lack an insulation shield. As a result, simply adding the typical amount of carbon black which yields UV protection, such as about 25 percent by weight, also makes the conductivity of the insulation too high so that the insulation loses its non-tracking properties. In order to achieve the required performance, a balance between UV protection and insulative properties, that is, dielectric strength is needed.
According to the present invention, a combination of polymer resin, antioxidant, carbon black and UV inhibitors, and a method of cross-linking same with silane, has been discovered which produces an insulation satisfying the physical, electrical and non-tracking properties set forth in FAA L-824 for airport lighting cables. Therefore, in one embodiment the present invention is a cable insulation designed to reduce the surface discharge of electrical current by giving the insulation enough conductivity so tracking does not occur but not so much that the insulation loses its insulative properties. Additionally, the insulation has enough resistance to weathering to prevent degradation. In another embodiment the invention is a cable comprising a conductor, a conductor shield and the non-tracking cable insulation. In yet another embodiment the invention is a method of making cross-linked, non-tracking electrical cable insulation.
According to the related art, 5 kV track resistant, non-shielded cable can be made using commercially available peroxide cross-linked polymers on a continuous vulcanization (xe2x80x9cCVxe2x80x9d) line. Continuous vulcanization is a continuous, in-line process whereby a wire has an extruded covering applied and is then passed through a tube containing such temperatures and pressures as are necessary to complete cross-linking. Commercially available non-tracking insulation materials of this type includes, for example, LE 4219 from NOVA-Borealis which consists of low density polyethylene (xe2x80x9cLDPExe2x80x9d), carbon black (about 0.5 percent by weight), and peroxide. Also, PC 731 from NOVA-Borealis which consists of LDPE, calcium carbonate, peroxide, titanium dioxide and carbon black. Cables employing these insulations in conjunction with CV cured conductor shields such as HFDA-0581 from Union Carbide Corporation and LE 0595 from NOVA-Borealis are also known in the art. These materials, however, are cross-linked on continuous vulcanization lines via peroxide systems which limits the line speed at which cables incorporating this insulation may be manufactured.
U.S. Pat. No. 4,426,549 to Natwig discloses track and erosion resistant electrical insulating materials comprising a base insulating material of a polymer of ethylene and additives to allow the polymer to function as electrical insulation, and hydrated zinc borate as an anti-tracking and anti-erosion additive.
Further, U.S. Pat. No. 4,384, 944 to Silver et al. relates to irradiation cross-linked, polymeric insulation for electric cables which may be a polyethylene doped with carbon black. An exemplary embodiment of the invention comprises a power cable having a central conductor surrounded by two layers of semiconducting and insulating material. Both surrounding layers may comprise polymeric material such as polyethylene which is doped with carbon black of specific grades and in particular concentrations. Exterior armoring or shielding layers may also be present where required.
U.S. Pat. No. 4,399,060 to Glass teaches a semiconductive elastomeric compound comprising a blend of a copolymer of ethylene, alkyl acrylate and mono-alkyl ester of 1,4-butenedioic acid and a copolymer of ethylene with propylene and/or an unconjugated diene. The polymeric material is doped with conductive carbon black and is cured with a peroxide curing agent. Similarly, the polyethylene composions shown in U.S. Pat. No. 5,256,482 to Yamanouchi and U.S. Pat. No. 5,556,697 are cured with organic peroxide curing agents.
These references, however, do not suggest the advantages of the present invention and they have different formulations, applications, or cross-linking methods. The present invention uses a combination of linear low density polyethylene, carbon black and UV inhibitors (as opposed to mineral fillers) to meet the track resistant properties necessary for this application as airport lighting cables. Additionally, the present invention uses commodity resins and cross-linking via silane to significantly reduce the raw material costs. Processing costs also are decreased because the present invention is a moisture cured product capable of being produced at line speeds of about 300 feet per minute as opposed to a CV cured product which generally operates at a significantly lower line speed of about 8 feet per minute.
U.S. Pat. No. 5,824,718 to Penfold et al. relates to silane crosslinkable, substantially linear ethylene polymers and their use as electrical cable insulation. Cable insulations made according to this invention are stated to have enhanced tree resistance, heat resistance, abrasion resistance, flexibility and cure under ambient conditions. Similarities between Penfold and the present invention are that both employ silane crosslinking. However, Penfold uses homogeneously branched, substantially linear ethylene polymers as contrasted with the linear low density polyethylene of the present invention which is a conventional heterogeneously branched polyolefin resin prepared with a coordination catalyst. Furthermore, Penfold does not contemplate, teach or suggest a track resistant insulation nor does this reference teach or suggest the use of carbon black or UV inhibitors to achieve track resistance.
Accordingly, one object of the present invention is to provide a track resistant insulation for non-shielded electrical power cables which complies with the ICEA S-66-524 and FAA L-824 technical specifications. Another object of the invention is to provide a cost-effective, track resistant insulation by using low cost materials. A still further object of the invention is to provide an electric cable employing a track resisting insulation material which can be manufactured at a significantly faster line speed than a cable employing a track resistant insulation material manufactured by a continuous vulcanization process.
These and other objects and advantages are obtained in accordance with the present invention by forming a silane cross-linked track resistant insulation material for an electrical cable comprising a base resin of linear low density polyethylene (xe2x80x9cLLDPExe2x80x9d). Non-tracking and weathering properties are introduced to the base resin via a black masterbatch containing a polymer (such as LLDPE, low density polyethylene or poly (ethylene-co-vinylacetate)) having a melting temperature less than or equal to the melting temperature of the LLDPE base resin, an antioxidant, conductive carbon black and ultraviolet (xe2x80x9cUVxe2x80x9d) ray inhibitors. The antioxidant and UV inhibitor may optionally be included in the LLDPE base resin. Exemplary antioxidants are hindered phenols such as tetrakis [methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]-methane, bis[(beta-(3,5-ditert-butyl-4-hydroxybenzyl)methylcarboxyethyl)]sulphide, 4,4xe2x80x2-thiobis(2-methyl-6-tert-butylphenol), 4,4xe2x80x2-thiobis(2-tert-butyl-5-methylphenol), 2,2xe2x80x2-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate; thio compounds such as dilaurylthiodipropionate, dimyristylthiodipropionate, and distearylthiodipropionate; various siloxanes; and various amines such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline. Antioxidants can be used in an amount of about 0.05 to about 2 weight percent of the total composite insulation material. Additionally, a metal deactivator such as IRGANOX 1035 (thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate) may optionally be included in the black masterbatch or in the LLDPE base resin. The use of a metal deactivator is highly preferred when the central conductor of the electrical power cable is copper.
Herein xe2x80x9csilanexe2x80x9d is understood to mean any silane composition which includes a crosslinking initiator and a catalyst. Preferably the silane used in the present invention is a mixture of vinyltrimethoxy silane, dicumyl peroxide and dibutyltin dilaurate such as DYNASYLAN(copyright) Silfin-06 from Sivento Chemie GmbH or SILCAT R(copyright) from Witco Corporation. On a macroscopic level the LLDPE base resin and the polymer of the black masterbatch are cross-linked via the silane composition. That is, the antioxidant, carbon black, metal deactivator and UV inhibitor are not integral parts of the cross-link structure. However, cross-links can terminate on the antioxidant, carbon black, metal deactivator and UV inhibitor to a small degree and therefore these ingredients of the insulation material of the present invention can be part of the cross-link structure.